A player is All-in when they have committed all of their remaining chips to the pot, after which the hand continues should there be any other players left to act. If we are playing in a multi-table tournament, have 10,000 chips left, ‘shove’ all-in for our whole stack and are called by an opponent with a 15,000 stack, then they have us ‘covered’ and can eliminate us from the tournament.
The potential implications of moving all-in (or ‘pushing’, ‘jamming the pot’ etc) differ depending on the game format. For instance, if the example above were a No Limit Hold’em Freezeout tournament, losing to an opponent with more chips in an all-in scenario means being knocked out altogether, with no option to continue. In a ‘rebuy’ tournament we see more all-ins because players have the option of rebuying (or re-entering) for a new starting stack. Meanwhile, in a Cash Game, we can go all-in, lose our stack, and simply buy back into the game.